Dome sheets are widely used in the manufacture of electronic keyboards for many applications, including computer keyboards. As computer keyboards have become larger, due to inclusion of auxiliary keyswitches and pads, the underlying dome sheets used as keyswitch springs and/or switch element actuators have also progressed in size. The larger areas of such dome sheets pose a challenge in their manufacture, which involves molding elastomeric rubber or plastic material. The nature of the elastomeric materials and molding procedures used in manufacturing dome sheets has sometimes resulted in the production of dome sheets having less precision than their mating plastic parts in a keyboard assembly. The resulting variations in size create assembly difficulties as the dome sheets are mated to adjacent parts that must be aligned with the domes contained on them.
When individual domes on a sheet are misaligned with their mating keyswitch actuators or underlying switching elements in a keyboard, the result ies a degraded feel or touch. Because of this, some keyboard manufacturers currently design dome sheets in two or more separate parts so that the individual parts can be properly aligned with the keyswitch assemblies during assembly of a keyboard. This results in extra handling during keyboard assembly and increases the part count and cost for each keyboard.
Another approach that has been attempted to assure proper alignment of molded dome sheets involves inclusion of thin sections across the sheets to divide them into two separably positionable sections by a membrane lying within the plane of the sheet itself. When pulled, such a membrane tends to stretch the adjacent portions of the attached sheet sections. When pushed, it tends to buckle the adjacent edges of the attached sheet sections. Such thin sections within the plane of the sheet are also difficult to properly mold and have been found to have a tendency to tear during assembly.
Illustrative examples of dome sheets can be found in U.S. Pats. Nos. 4,560,845 and 4,571,466, each of which is hereby incorporated into this disclosure by reference. The design and manufacture of dome sheets is well-known, and further details concerning their general features are not believed necessary to an understanding of the present invention.
The present disclosure pertains to an expansion joint that effectively divides a dome sheet into two or more sections. This permits the manufacture of keyboards to accomodate normal molding tolerances encountered during the production of such sheets, particularly at the larger sizes required by modern computer keyboards. It eliminates distortion of the domed sheet itself during keyboard assembly and reduces tolerance accumulation, typically a percentage of the length of the part, to an acceptable level.